The third anniversary of the Deepwater Horizon blowout seems a good time to take stock of the damage done to the Gulf of Mexico, and to look to its future.

The drill rig sank in about a mile of water on April 22, 2010, Earth Day, spewing more than 200 million gallons of crude oil into the Gulf. It took almost a hundred days to cap the underwater well. During that time, shifting currents spread oil-based toxic substances far and wide, at many depths in the sea, exposing many forms of sea life to potentially deadly pollution.

Today, storms still churn up oil and perhaps one third of the toxic materials from the blowout remain loose in the Gulf. Scientists are still struggling to estimate the damage done.

Visible and Invisible Damage

From the spill’s first days, the public focus was on direct human effects, on animals that were being visibly “oiled” – sea birds, dolphins and whales – and on coastal habitats onto which the oil might wash. When drifting oil made landfall on beaches and in saltmarshes, that was big news. But the worst impacts of the spill remained largely out-of-sight and out-of-mind.

The blown-out well is located in deepwater right at the edge of the continental shelf, in close proximity to ancient and highly vulnerable deepwater coral reefs. These were bathed in organic pollutants for months, and surveys have revealed devastation in those coral communities, including serious damage to corals that take thousands of years to grow. It will take centuries – or longer – for those reefs to recover.

Moreover, toxicants now cover a wide range of seafloor habitats, where they are being consumed and processed by myriad creatures that make their living by eating sediments and digesting organic matter found there. These small creatures, carrying their poisonous payloads then enter food webs when they are eaten by fish. This will be a long-term problem with no easy solution. Recent reports from the field of eyeless shrimp and fish covered with sores suggest that significant amounts of toxic substances are indeed still loose in the Gulf food chain.

Damage also was concentrated in the middle depths of the sea. This so-called “deep scattering layer” is home to swarms of small animals, so dense that they reflect radar signals. This abundant soup of life is the food target zone for deep-diving fishes and mammals – tunas and billfishes, dolphins and great whales. The deep-scattering layer is especially important in the “spill kill zone” of the Northern Gulf, where a large population of sperm whales lives.

Deep scattering layer animals were very likely obliterated for a wide distance around the well site by vast underwater plumes of dissolved and dispersed oil-derived toxicants, nearly certainly made worse by the use – including underwater – of two million gallons of dispersants. The dispersant/oil mixture has since been shown to be up to fifty times as toxic for many sea creatures as the oil by itself. Dispersant use certainly spread the toxic brew much more broadly under the sea.

On the surface, birds and other large animals were oiled and killed, but immeasurably greater numbers of floating baby sea creatures died as well. The oil – and the toxic dispersant brew – covered large areas of sea surface, where buoyant eggs and larvae drift, between the spawning areas at the edge of the coastal shelf and the coastal nurseries (wetlands, tidal creeks and even sandy beaches). For many key species, those larvae drifted through during the peak of the spill.

In addition, the Gulf Loop Current acts as a sea highway for drifting larvae of Atlantic Bluefin tuna, groupers, snappers spiny lobsters and a host of other species spawned far upcurrent. These larvae, too, drifted through the BP “kill zone” during the time of the spill.

Oil-Eating Bacteria?

As the spill progressed, many observers expressed great relief that the oil stayed offshore, and that it was apparently consumed by microorganisms, including “oil-eating” bacteria that were said to have eliminated the threat. In truth, however, there is no free lunch, even in the sea. It’s true that populations of naturally occurring but normally rare microorganisms exploded, using oil as a feed-stock. This altering ecosystem conditions along many lines. The explosion in bacteria sucked oxygen from the sea and liberated carbon dioxide, deoxygenating and acidifying mid-depth oceans waters and altering food webs in ways that may never be fully quantified.

The Spill’s Bottom Line

The damage done to sea life by the blowout cannot be quantified today. No one knows how many baby reef fish and lobsters died, or how those losses will affect regional fisheries. It may turn out to be possible to make an informed estimate down the road, if extensive modeling is done to compare the numbers of adults that came from 2010 spawning to what might have been expected for at least some of the most economically important animals (perhaps red snapper and some groupers).

I suspect that serious reductions in so-called “year class strength” – the relative number of animals reaching the size where they enter fisheries – occurred for many important species, like shrimp, blue crabs, menhaden and others.

Still, with so much unknown, providing economic loss estimates for the damage done by the spill is challenging. Besides, how do you value pelicans, sea turtles, and dolphins? How do you value – much less replace – a million-year-old coral mound?

When one puts all of these pieces together, it is clear that the Gulf took a real body blow, probably much worse than people understand. Fortunately, there are forces at work that provide hope for the future of the Gulf. I’ll address some of these forces in my next post, but until then you can help make BP pay.